An image sensor that has heretofore been proposed for use in reading image on facsimile or other recording equipment is of such a design that one light-receiving element is formed of two photodiodes that are connected in series in such a way that the relationship of polarities of one photodiode is reverse to that of the other photodiode and that a plurality of such light-receiving elements are arranged in a lineal or matrix pattern. An exemplary structure of this type of image sensor as shown in FIGS. 10A and 10B has already been proposed in U.S. patent application Ser. No. 07/638,983. FIG. 10A is a plan view of the image sensor and FIG. 10B is a cross section of FIG. 10A taken on line J--J. As shown, one end of a photodiode PD3 is connected to a common electrode wiring 51 whereas one end of a photodiode PD4 is connected to an individual electrode wiring 53 via a lead wiring 52. Photodiode PD3 is formed in the area where a photoelectric transducing layer 60a is held between a transparent electrode 61a and a metal electrode 62, whereas photodiode PD4 is formed in the area where a photoelectric transducing layer 60b is held between a transparent electrode 61b and the metal electrode 62. The metal electrode 62 and the individual electrode wiring 53 are both composed by forming a metal layer such as chromium on a transparent substrate 70 typically made of glass.
An outline of the procedure for reading signals with the image sensor having the structure shown in FIG. 10 is described below with reference to FIG. 11. First, a series of photodiodes PD3 are scanned with a shift register SR so that they are sequentially supplied with a reverse bias voltage. As a result, electric charges are stored in the photodiodes PD3. After the scanning step, the charges stored in the photodiodes PD3 are distributed between the capacitance C.sub.P3 of the photodiodes PD3 and the capacitance C.sub.P4 of the photodiodes PD4. As one cycle of the scanning step is completed, the photodiodes PD3 and PD4 are illuminated with light, causing discharge in an amount that depends on the quantity of light illumination. In the next step, reset signals which are readout pulses are sequentially applied by means of the shift register SR, causing the photodiodes PD to be recharged in an amount that depends on the amount of previous discharge. As a result, a current flows through a loading resistor R, producing a potential at an output terminal T.sub.OUT. An output signal can be obtained by reading this potential as a signal. Hence, the image sensor under consideration has the advantage that even a two-dimensional array of light-receiving elements can be driven with a driver circuit for one line.
The image sensor of the construction shown in FIGS. 10A and 10B has had the following problems. FIG. 12 is the same as FIG. 10B except that two areas A and B are hatched. As indicated by hatched area A, the photodiode PD3 has a capacitance of no light reception between the metal electrode 62 and the transparent electrode 61a (which capacitance is hereunder referred to as "additional capacitance") in the region that lies under the common electrode wiring 51 to be shielded from light; further, as indicated by hatched area B, photodiode PD4 has an additional capacitance between the metal electrode 62 and the transparent electrode 61b in the region that lies under the lead wiring 52 to be shielded from light. Because of those additional capacitances, the response of the photodiodes deteriorates and, hence, their output voltages will decrease to cause the residual image problem.
This phenomenon may be explained as follows with respect to one bit of the image sensor, or one light-receiving element. When photodiode PD4 is supplied with a readout pulse of a duration t.sub.r, the anode of PD4 remains connected to a reset voltage V for the period t.sub.r, (see FIG. 13), whereas the anode is grounded when no readout pulse is applied. Thus, upon application of a readout pulse, charges (Q=C.sub.P3 .multidot.V) are stored in the photodiode PD3 which is connected in reverse direction with respect to reset voltage V. When the readout pulse is no longer applied, the charges Q (=C.sub.P3 .multidot.V) stored in the photodiode PD3 are distributed in accordance with the ratio of C.sub.P3 to C.sub.P4, whereby C.sub.P3 .multidot.Q/(C.sub.P3 +C.sub.P4) is assigned to the photodiode PD3 whereas C.sub.P4 .multidot.Q/(C.sub.P3 +C.sub.P4) is assigned to the photodiode PD4.
In the next step, light is incident on the photodiodes PD3 and PD4 (the storage time is t.sub.a). If the photo currents generated in the photodiodes PD3 and PD4 are represented as i.sub.3 and i.sub.4, respectively, charges (.DELTA.q.sub.3 =i.sub.3 .multidot.t.sub.a) will be generated in photodiode PD3 whereas charge (.DELTA.q.sub.4 =i.sub.4 .multidot.t.sub.a) will be generated in photodiode PD4; the thus generated charges will be recombined with the charges already stored in the photodiodes PD3 and PD4, whereby the charge will be re-distributed between the photodiodes PD3 and PD4 in accordance with the ratio determined by the following equations (1) and (2): EQU PD3: C.sub.P3 .multidot.(Q-i.sub.3 .multidot.t.sub.a -i.sub.4 .multidot.t.sub.a)/(C.sub.P3 +C.sub.P4) (1) EQU PD4: C.sub.P4 .multidot.(Q-i.sub.3 .multidot.t.sub.a -i.sub.4 .multidot.t.sub.a)/(C.sub.P3 +C.sub.P4) (2)
If, in the next step, a readout pulse is applied again, the voltage determined by the following equation (3) will be detected at the output terminal T.sub.OUT : EQU C.sub.P4 .multidot.Q/(C.sub.P3 +C.sub.P4)+C.sub.P3 .multidot.(i.sub.3 .multidot.t.sub.a +i.sub.4 .multidot.t.sub.a)/(C.sub.P3 +C.sub.P4)(3)
The first term of eq. (3) denotes a dark output, or an output that is produced when there is no incident light and, therefore, the effective output will be expressed by the second term of eq. (3).
In practice, however, both photodiodes PD3 and PD4 form an additional capacitance--in the case of PD3, the additional capacitance is formed in the region shielded from light by the common electrode wiring 51 and in the case of PD4, it is formed in the region shielded from light by the lead wiring 52. Because of those additional capacitances, the photodiode PD3 will be reset before an adequate amount of charges are stored across the two ends while a readout pulse is being applied. As a result, the output of PD3 will decrease to produce a residual image.